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Re: [Qemu-block] [PATCH 4/4] block: Cater to iscsi with non-power-of-2 d

From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [PATCH 4/4] block: Cater to iscsi with non-power-of-2 discard
Date: Tue, 26 Jul 2016 14:28:02 +0100
User-agent: Mutt/1.6.2 (2016-07-01)

On Thu, Jul 21, 2016 at 01:34:48PM -0600, Eric Blake wrote:
> Dell Equallogic iSCSI SANs have a very unusual advertised geometry:
> $ iscsi-inq -e 1 -c $((0xb0)) iscsi://XXX/0
> wsnz:0
> maximum compare and write length:1
> optimal transfer length granularity:0
> maximum transfer length:0
> optimal transfer length:0
> maximum prefetch xdread xdwrite transfer length:0
> maximum unmap lba count:30720
> maximum unmap block descriptor count:2
> optimal unmap granularity:30720
> ugavalid:1
> unmap granularity alignment:0
> maximum write same length:30720
> which says that both the maximum and the optimal discard size
> is 15M.  It is not immediately apparent if the device allows
> discard requests not aligned to the optimal size, nor if it
> allows discards at a finer granularity than the optimal size.
> I tried to find details in the SCSI Commands Reference Manual
> Rev. A on what valid values of maximum and optimal sizes are
> permitted, but while that document mentions a "Block Limits
> VPD Page", I couldn't actually find documentation of that page
> or what values it would have, or if a SCSI device has an
> advertisement of its minimal unmap granularity.  So it is not
> obvious to me whether the Dell Equallogic device is compliance
> with the SCSI specification.
> Fortunately, it is easy enough to support non-power-of-2 sizing,
> even if it means we are less efficient than truly possible when
> targetting that device (for example, it means that we refuse to
> unmap anything that is not a multiple of 15M and aligned to a
> 15M boundary, even if the device truly does support a smaller
> granularity where unmapping actually works).
> Reported-by: Peter Lieven <address@hidden>
> Signed-off-by: Eric Blake <address@hidden>
> ---
> Help in locating the actual specs on what SCSI requires for
> page 0xb0 would be nice. But this should at least avoid the
> assertion failures that Peter is hitting.  I was able to
> test this patch using NBD on a hacked up qemu where I made
> block/nbd.c report the same block limits, and could confirm
> the assert under qemu-io 'w -z 0 40m' and 'discard 0 40m'
> pre-patch, as well as the post-patch behavior of splitting
> things to 15M alignment ('discard 1M 15M' becomes a no-op
> because it is not aligned).  But obviously it needs to be
> tested on the actual iscsi SAN that triggered the original
> report.
> ---
>  include/block/block_int.h | 37 ++++++++++++++++++++-----------------
>  block/io.c                | 15 +++++++++------
>  2 files changed, 29 insertions(+), 23 deletions(-)

Acked-by: Stefan Hajnoczi <address@hidden>

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